Electrical energy storage cells are widely used to provide power to electronic, electrical, electromechanical, electrochemical, and other useful devices. Such cells include primary chemical cells, secondary (rechargeable) cells, fuel cells, and various species of capacitors. Important characteristics of electrical energy storage cells include energy density, power density, charging rate, internal leakage current, equivalent series resistance (ESR), and ability to withstand multiple charge-discharge cycles. For a number of reasons, double-layer capacitors, also known as supercapacitors and ultracapacitors, are gaining prominence among the various electrical energy storage cells. These reasons include availability of double-layer capacitors with high power densities, and energy densities approaching those of conventional rechargeable cells.
Double-layer capacitors use electrodes immersed in an electrolyte (an electrolytic solution) as their energy storage element. Typically, a porous separator soaked in the electrolyte ensures that the electrodes do not come in contact with each other, thus preventing electronic current flow directly between the electrodes. The porous separator allows ionic currents to flow between the electrodes in both directions. As discussed below, double-layers of charges are formed at the interfaces between the solid electrodes and the electrolyte.
When electric potential is applied between a pair of electrodes of a double-layer capacitor, ions that exist within the electrolyte are attracted to the surfaces of the oppositely-charged electrodes, and migrate towards the electrodes. A layer of oppositely-charged ions is thus created and maintained near each electrode surface. The electrical energy is stored in the charge separation layers between these ionic layers and the charge layers of the corresponding electrode surfaces. The charge separation layers behave essentially as electrostatic capacitors. Electrostatic energy can also be stored in the double-layer capacitors through orientation and alignment of molecules of the electrolytic solution under influence of the electric potential.
Although double-layer capacitors may store additional energy through electrochemical processes, such effects are secondary, accounting for less than ten percent of the electrostatically stored energy in a typical double-layer capacitor.
In comparison to conventional capacitors, double-layer capacitors have high capacitance in relation to their volume and weight. There are two main reasons for this volumetric and weight efficiency. First, the charge separation layers are very narrow. Their width are typically on the order of nanometers. Second, the electrodes can be made from a porous material, having very large effective surface area per unit volume. Because capacitance is directly proportional to the electrode area, and inversely proportional to the width of the charge separation layer, the combined effect of the narrow charge separation layer and large effective surface area results in a capacitance that is very high in comparison to that of conventional capacitors. High capacitance of the double-layer capacitors allows the capacitors to receive, store, and release large supplies of electrical energy.
Another important performance characteristic of a capacitor is its internal equivalent series resistance. Frequency response of a capacitor depends on the characteristic time constant of the capacitor, which is essentially a product of the capacitance and the equivalent series resistance, or RC. To put it differently, equivalent series resistance limits both charge and discharge rates of a capacitor, because the resistance limits the current flow into or out of the capacitor. Maximizing the charge and discharge rates is important in many applications, such as automotive applications. Excessive internal resistance also dissipates energy during both charge and discharge cycles, converting the electric energy into heat. This reduces capacitor efficiency, and may affect both durability and reliability. Therefore, equivalent series resistance should be kept reasonably low.
Electrical energy storage capability of a capacitor can be determined using a well known formula, to wit:
                              E          =                                    C              *                              V                2                                      2                          ,                            (        1        )            in which E is the stored energy, C is the capacitance, and V is the voltage of the charged capacitor. Thus, the maximum energy (Em) that can be stored in a capacitor is given by the following expression:
                                          E            m                    =                                    C              *                              V                r                2                                      2                          ,                            (        2        )            where Vr stands for the rated voltage of the capacitor. It follows that a capacitor's energy storage capability depends on both (1) its capacitance, and (2) its rated voltage. Increasing these two parameters is therefore important to capacitor performance. Indeed, because energy capacity varies as a second order of the voltage rating, while varying linearly with capacitance, increasing the voltage rating is the more important goal for capacitors in many applications. With increases in voltage rating, it is understood that increases in power density may also be achieved.
Voltage ratings of double-layer capacitors are generally limited by chemical reactions (reduction, oxidation) and breakdown within the electrolytic solutions in the presence of electric field generated between electrodes. Electrolytic solutions currently used in double-layer capacitors are of two kinds. The first kind of electrolytic solutions includes organic solutions, such as propylene carbonate and Acetronitrile.
The second kind of double-layer electrolytic solutions includes aqueous solutions, such as potassium hydroxide and sulfuric acid solutions.
Both reduction and oxidation processes can be identified from what is known to those skilled in the art as a CV curve, which may obtained by cyclically applying a varying voltage to a double-layer capacitor and measuring a resulting capacitance.
Organic electrolyte may typically enable higher operating voltage as compared to aqueous electrolyte, however, because of the conductivity of organic electrolytes, equivalent series resistance may be higher.
Whatever the nature of the specific double-layer capacitors or other energy storage cells of interest, it would be desirable to increase their voltage ratings.
As used later for comparisons, a comparative double-layer capacitor manufactured with active electrode films of similar thickness was tested. In one embodiment, the thickness of the electrode films was about 100 microns. A particular activated carbon used in manufacture of the activated electrode film for use in a double-layer capacitor was a type YP-17 sold by Kuraray Chemical Co., LTD, Shin-hankyu Bldg. 9F Blvd. C-237, 1-12-39 Umeda, Kiata-ku, Osaka 530-8611, Japan. Empirical test data from the comparative double-layer capacitor was obtained, and indicated that performance of the capacitor decreased as the operating voltage of the capacitor was increased above 2.5 volts. For example, the number of times the comparative double-layer capacitor could be charged and discharged was seen to decrease with increased operating voltage.
A need thus exists for double-layer capacitors with enhanced energy storage capabilities. Another need exists for double-layer capacitors with increased voltage ratings. Yet another need exists for methods of designing and manufacturing double-layer capacitors with increased voltage ratings and enhanced energy and power density capabilities.